Biomedical subjects

S B Howell

Publications and source records attributed to S B Howell.

At least 109 records · Page 6Linked to original sources

PubMed1993

Modulation of cisplatin cytotoxicity by permeabilization of the plasma membrane by digitonin in vitro.

Killing of human ovarian carcinoma 2008 cells by cisplatin (DDP) is in direct proportion to the amount of drug entering the cell. DDP and its analogue [3H]dichloro(ethylenediamine)platinum[II] ([3H]-DEP) enter cells relatively slowly. We found that the uptake of [3H]DEP into 2008 cells could be increased by treating the cells briefly with the plasma membrane-selective detergent digitonin. A similar effect was observed in an 11-fold DDP-resistant subline of 2008 cells, designated 2008/C13*5.25. A measurable effect was produced by concentrations as low as 5 microM, and 40 microM digitonin increased [3H]DEP accumulation at 1 hr by 4.4 +/- 0.2- and 6.5 +/- 0.7-fold (means +/- SD) in 2008 and 2008/C13*5.25 cells, respectively. The effect was rapid, occurring within 1 min. Increased [3H]DEP uptake was accompanied by increased platination of DNA (8.5-fold in 2008 cells and 18.5-fold in 2008/C13*5.25 cells), and by enhanced killing of both the DDP-sensitive and -resistant cells that was shown to be synergistic by median effect analysis. The combination index at 50% cell kill was 0.64 +/- 0.14 (values < 1 indicate synergy). We conclude that a brief exposure to digitonin can increase [3H]DEP uptake in vitro, and can overcome the impaired [3H]DEP accumulation associated with acquired DDP resistance. DDP and digitonin interact synergistically to increase tumor cell kill in vitro.

Antineoplastic Agents↗

PubMed1993

Tamoxifen modulation of cisplatin sensitivity in human malignant melanoma cells.

Tamoxifen (TAM) markedly increases the response rate of malignant melanoma to treatment with cisplatin (DDP), carmustine, and dacarbazine, and we have previously reported that there is a highly synergistic interaction between TAM and DDP with respect to the cytotoxic effect against the human melanoma cell line T-289 (E. F. Mc Clay et al., Cancer Res., 52: 6790-6796, 1992). The mechanism underlying synergy was investigated by examining the effect of selection for either DDP or TAM resistance on the magnitude of the synergy quantitated by median effect analysis. The combination index at 50% cell kill was 0.26 +/- 0.02 (SD) for parental T-289 cells (indicating marked synergy), 0.54 +/- 0.14 for cells selected for low-level DDP resistance (indicating moderate synergy), and 1.39 +/- 0.20 for cells selected for low-level TAM resistance (indicating antagonism). Thus, factors that regulate DDP sensitivity have a moderate effect on reducing the DDP/TAM synergy, but determinants of TAM sensitivity have a major effect. The known biochemical effects of TAM include antagonism of estrogen at the estrogen receptor (ER) and inhibition of calmodulin and protein kinase C activity. T-289 cells contained undetectable amounts of ER by the dextran-coated charcoal assay and expressed only trace amounts of ER mRNA, and another more avid ER antagonist, droloxifene, failed to interact synergistically with DDP. N-(6-Aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), a potent calmodulin antagonist, failed to demonstrate synergy with DDP, and activation of protein kinase C, instead of interacting antagonistically with DDP, yielded synergy. TAM did not alter the cell cycle phase perturbation produced by exposure to DDP alone. We conclude that the synergy between TAM and DDP is not mediated by the effects of TAM on the ER, calmodulin, protein kinase C, or cell cycle regulation. However, the factors that determine cellular sensitivity to TAM also determine whether TAM interacts synergistically with DDP.

Antineoplastic Agents↗

PubMed1993

Prolongation of drug exposure in cerebrospinal fluid by encapsulation into DepoFoam.

Prolonged maintenance of a therapeutic drug concentration in the cerebrospinal fluid is required for optimal treatment of leptomeningeal leukemia or carcinomatosis with cell cycle-specific antimetabolites. The pharmacokinetics of 1-beta-D-arabinofuranosylcytosine (ara-C) encapsulated into DepoFoam (Depo/Ara-C) was studied in six rhesus monkeys after intrathecal injection into the lumbar sac. Following a single 2-mg dose, the Depo/Ara-C concentration decreased biexponentially with initial and terminal half-lives of 14.6 and 156 h, respectively. The free drug concentration remained above the reported minimal cytotoxic level of 0.1 micrograms/ml (0.4 microM) for more than 672 h (28 days). In contrast, the half-life of ara-C following an intralumbar bolus dose of unencapsulated drug in a single animal was 0.74 h. A single intrathecal injection of Depo/Ara-C can maintain a therapeutic drug concentration in the cerebrospinal fluid for a very prolonged period.

Animals↗

PubMed1993

Treatment of leptomeningeal metastasis with intraventricular administration of depot cytarabine (DTC 101). A phase I study.

Depo cytarabine (DTC 101 [formerly identified as Depo/Ara-C]) is a slow-releasing, depot formulation in which cytarabine is encapsulated within the aqueous compartments of microscopic (DepoFoam) particles. A phase I trial of DTC 101, given intraventricularly, was conducted in patients with leptomeningeal metastasis. Nine patients were given 1 to 7 cycles of DTC 101 in doses ranging from 25 to 125 mg that were administered via an Ommaya reservoir into the lateral ventricle. The dose-limiting toxic reaction was encephalopathy that occurred at the 125-mg dose level. All toxic episodes but one were transient and reversible, with the total duration of toxicity lasting from 1 to 7 days. The ventricular concentration of free cytarabine released from DTC 101 into cerebrospinal fluid decreased biexponentially with an initial half-life of 7.2 +/- 1.7 (+/- SEM) hours and a terminal half-life of 140 +/- 49 hours. The cerebrospinal fluid was cleared of malignant cells within 3 weeks of initial therapy in five of six cytologically evaluable patients. The duration of response ranged from 2 to more than 14 weeks, with a median of over 11 weeks. In conclusion, DTC 101 appears to be a pharmacologically attractive agent for use against leptomeningeal metastasis. The toxic episodes that occur with this therapy are well tolerated by patients.

Adult↗

PubMed1993

High-dose biweekly intraperitoneal cisplatin: an effective way to increase cisplatin dose intensity.

The amount of cisplatin (DDP) delivered per unit time (dose intensity, expressed in mg/m2/week) may be an important factor in determining the clinical outcome in tumors such as ovarian carcinoma. In this neoplasm, intraperitoneal chemotherapy is an effective form of treatment. In this trial we have explored the tactic of shortening the cycle interval as a way to increase the dose intensity of ip DDP. Sixteen patients with a variety of solid tumors received a total of 77 cycles of DDP 180 mg/m2 instilled ip concurrent with i.v. sodium thiosulfate at the dose of 4 g/m2 as loading dose, followed by 12 g/m2 over 6 hr. Each cycle was repeated every 2 weeks. The number of cycles delayed for 3 or more days was 28 (36%). The mean DDP dose intensity received by these patients was 77% of the planned dose or 69 mg/m2/week (confidence interval 95%, 60.5-77.5). The treatment was generally well tolerated: myelotoxicity was mild, only 1 patient had an increase in serum creatinine to > 2 mg/dl. Five patients (31%) developed symptoms of peripheral neuropathy. All patients were evaluable for response. The overall response rate (complete plus partial) in these heavily pretreated patients was 19%. When DDP is given in high doses by the ip route concurrently with systemic sodium thiosulfate, the dosing interval can be reduced to every 2 weeks permitting a marked increase in DDP dose intensity.

Abdominal Neoplasms↗

PubMed1993

Phase I and pharmacokinetic study of intraperitoneal ormaplatin.

Ormaplatin is a cisplatin analog which has demonstrated activity against cisplatin-resistant tumors in preclinical studies. We delivered 28 cycles to 14 patients in a phase I trial of intraperitoneal ormaplatin given every 28 days. The maximum tolerated dose was 88.4 mg/m2 and acute dose-limiting toxicity was abdominal pain. Other toxicities include nausea, emisis, fever, and severe neuropathy seen in 1 patient at a cumulative dose of 399 mg/m2. No objective responses were observed. Hematologic toxicity was mild. The dose recommended for future trials of intraperitoneal ormaplatin is 66.5 mg/m2. Pharmacokinetic analysis performed at a dose of 66.5 mg/m2 demonstrated that the initial phase of elimination from the peritoneal cavity follows first-order kinetics with k = 0.69 hr-1 and half-life of 1.4 hr. Plasma pharmacokinetic behavior is best described by biexponential model with k1 = 0.369 hr-1, k2 = 0.107 hr-1, and first half-life of 2.9 hr and second half-life of 8.4 hr. Pharmacologic advantage, calculated by ratio of peritoneal to plasma AUC, is 17.1. If site-specific activity is demonstrated, then the intraperitoneal route of administration of ormaplatin at 66.5 mg/m2 may be beneficial.

Adult↗

PubMed1993

Analysis of the cytotoxic interaction between cisplatin and hyperthermia in a human ovarian carcinoma cell line.

Expression of the heat-shock protein HSP-60 is associated with poor survival in patients with ovarian carcinoma. We examined both the nature of the interaction between hyperthermia and cisplatin (DDP) using the human ovarian carcinoma cell line 2008 and the effect on this interaction of the induction of the heat-shock response. The nature of the interaction was assessed using median-effect analysis. Despite the observation that 45 degrees C hyperthermia increased the intracellular uptake of the DDP analog [3H]-cis-dichloro(ethylenediamine)platinum(II) (DEP) during a 1-h exposure by 155% +/- 55% (P = 0.02), median-effect analysis indicated only cytotoxic additivity (combination index at the level of 50% cell kill, 0.96 +/- 0.25). When cells were first exposed to hyperthermia for various periods and then allowed to incubate at 37 degrees C for 4 h to allow induction of the heat-shock genes before being treated with DDP for 1 h, there was a very small degree of antagonism between hyperthermia and DDP (combination index at 50% cell kill, 1.11 +/- 0.04). Our results indicate that DDP and hyperthermia interact only in an additive manner against this human ovarian carcinoma cell line and that the induction of heat-shock proteins by hyperthermia does not significantly antagonize the activity of DDP.

Cell Survival↗

PubMed1993

Regulation of HSP60 mRNA expression in a human ovarian carcinoma cell line.

The expression of the 60-kDa heat-shock protein (HSP60) varies markedly among patients with ovarian carcinoma, and high-level expression predicts poor survival in such patients treated with cisplatin (DDP)-containing chemotherapy programs. We investigated the expression of HSP60 in human ovarian carcinoma 2008 cells and an 11-fold DDP-resistant subline 2008/C13*5.25. Heating for 2 h at 44 degrees C produced a 2.7 +/- 0.16-fold increase (mean +/- SD) that was maximal at 4 h after the start of heat exposure. Exposure to an IC50 concentration of DDP for 1 h induced a 1.8 +/- 0.03-fold increase in hsp60 expression. The opposite was true for cadmium and zinc, both of which induced increases in metallothionein IIA but not in the hsp60 message. 2008/C13*5.25 cells constitutively over-expressed hsp60 mRNA by 1.7 +/- 0.16 orders of magnitude and contained a 3.8 +/- 0.45-fold higher level of HSP60 as detected by immunocytochemical staining. 2008/C13*5.25 cells showed 1.2-fold cross-resistance to thermal killing. Expression of hsp60 was markedly reduced in 2008 xenografts as compared with 2008 cells growing in vitro; however, neither serum starvation nor refeeding altered the message level. Exposure to a variety of growth factors and drug treatments known to alter the DDP sensitivity of 2008 cells, including epidermal growth factor, 12-O-tetradecanoylphorbol-13-acetate, buthionine sulfoximine, ouabain, and forskolin, did not alter hsp60 expression. These results suggest a role for HSP60 in mediating resistance to both DDP and hyperthermia but indicate that the hsp60 mRNA levels are not regulated by the factors listed above.

Blotting, Northern↗

PubMed1993

Regional lymphatic drug exposure following intraperitoneal administration of 5-fluorouracil, carboplatin, and etoposide.

Intraperitoneal (i.p.) administration of chemoterapeutic agents results in greater total drug exposures in the peritoneal cavity than in plasma. A study on the drug exposure for i.p. lymphatics of pigs, receiving 5-fluorouracil (5-FU), etoposide (VP-16) and carboplatin (CBDCA) by the i.p. route was conducted. Drug concentrations in peritoneal fluid, plasma, and thoracic duct lymph were monitored over the ensuing 3 h. 5-FU appeared rapidly in thoracic duct, lymph and plasma. The lymph concentration declined after 20 min while the plasma concentration remained stable. CBDCA reached a stable concentration in lymph and plasma after 60 min. VP-16 peaked in the lymph after 20 min, whereas the plasma concentration continued to rise for 150 min; the peritoneal half-life for VP-16 was too long for clearance to be defined. Total drug exposure (AUC) was for 5-FU 5.7-fold greater for lymph than for plasma and for CBDCA equal in both compartments. VP-16 had a 2.1-fold higher AUC for lymph than for plasma. The results indicate that the i.p. route of administration results in a greater exposure of the lower thoracic duct lymph than the plasma to 5-FU, produces only a marginally increased exposure to VP-16, and results in no difference for CBDCA. The efficacy of 5-FU is a function of total drug exposure. The results reported provide a strong rationale for evaluating the adjuvant use of i.p. 5-FU in colorectal and gastric carcinoma.

Animals↗

PubMed1993

Cellular accumulation of the anticancer agent cisplatin: a review.

Acquired resistance to cisplatin (DDP) is a major clinical problem in the treatment of ovarian, testicular, and head and neck carcinomas; decreased accumulation of DDP is the most consistently observed alteration in resistant cells. It has been postulated that DDP enters the cell by passive diffusion based on the observations that DDP accumulation is proportional to the drug concentration, accumulation is not saturable, and that structural analogs of DDP do not inhibit accumulation. However, recent studies show that DDP accumulation can be specifically stimulated or inhibited by pharmacological agents and the activation of signal transduction pathways. This paper reviews the existing data on the mechanism of DDP accumulation and develops the postulate that some component of transport occurs through a gated ion channel.

Animals↗

PubMed1993

Regulation of initial vinblastine influx by P-glycoprotein.

P-glycoprotein (PGP) is an energy-dependent efflux pump that serves to protect cells against the cytotoxicity of many natural product drugs including vinblastine (VBL). In this study we investigated the role of PGP in regulating initial VBL influx. The apparent influx of VBL, measured over the first 20 s, was 2-fold lower in KB-GRC1 cells expressing a transfected mdr1 gene at high level than in non-expressing parental KB-3-1 cells. Inhibition of PGP efflux function with dipyridamole increased the influx rate constant by 4.0-fold in the KB-GRC1 cells but only 2.1-fold in the KB-3-1 cells. Verapamil, another inhibitor of PGP-mediated efflux, increased the initial influx rate constant by 2.7-fold in the KB-GRC1 cells but only 1.4-fold in the KB-3-1 cells. Inhibition of PGP function by depletion of ATP increased influx by 6.8-fold and 2.2-fold in the two cell types, respectively. Mutation of PGP at both ATP binding sites abolished its ability to limit initial influx. Thus, VBL is serving as an efficient substrate for the efflux pump even within the first few seconds of drug exposure, consistent with the hypothesis that PGP may directly efflux drug from the cell membrane.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1993

Selective modulation of vinblastine sensitivity by 1,9-dideoxyforskolin and related diterpenes in multidrug resistant tumour cells.

The ability of 1,9-dideoxyforskolin (DDF), 1-deoxyforskolin (DF) and forskolin to modulate cellular sensitivity to vinblastine (VBL) was examined in drug-sensitive parental KB-3-1 cells and a multidrug-resistant subline, KB-GRC1, derived by transfection of mdr1. Fifty microM DF and forskolin enhanced the 1 h uptake of VBL by 8.0 +/- 0.7 (s.d.) and 4.7 +/- 2.5-fold, respectively, with 50 microM DDF producing a 13.6 +/- 1.9-fold increase. The greater effect of DDF relative to forskolin indicated that the effect was independent of activation of cAMP, and this was supported by a lack of effect of dibutyryl cAMP on the uptake. The effect of these agents on uptake were < or = 1.4-fold in KB-3-1 cells. DDF selectively inhibited initial efflux in cells expressing a functional P-glycoprotein (PGP), but both forskolin and DDF inhibited the terminal phase of efflux irrespective of PGP expression. Neither agent affected membrane permeability of polarisation and forskolin did not enhance the uptake of VBL in protein-free liposomes. At a non-toxic concentration of 20 microM, DDF and forskolin decreased the IC50 of VBL from 18.9 to 2.7 and 13 nM in KB-GRC1 cells, respectively, and DDF acted synergistically with VBL as shown by median effect analysis [combination index = 0.20 +/- 0.05 (s.d.)]. In contrast, these diterpenes did not affect VBL sensitivity in KB-3-1 cells. These results indicate that the diterpenes modulate VBL sensitivity predominantly by inhibiting PGP-mediated efflux activity.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMed1993

Dose intensification of cisplatin chemotherapy through biweekly administration.

PURPOSE: Dose intensity (DI, expressed in mg/m2/wk) may be an important factor in the clinical use of cisplatin (DDP). We have explored the shortening of the cycle interval as a way to increase the DI of DDP. PATIENTS AND METHODS: DDP 180 mg/m2 was given intravenously (i.v.) over 4 hours; sodium thiosulfate (STS) was given i.v. in the opposite arm at a loading dose of 4 g/m2, followed by 12 g/m2 over 6 hours. Each cycle was repeated every two weeks. Seventy-five cycles were administered to 28 patients in this clinical trial. RESULTS: In 19 patients who received 2 or more cycles of chemotherapy, a delay of three or more days was required on 17/66 courses (26%); the mean DDP DI actually received by these patients was 83 mg/m2/wk (88% of the planned DI). The major side effect was ototoxicity; this occurred in 9 patients (33%), but none required a hearing aid. Myelosuppression was moderate with thrombocytopenia greater than neutropenia. Nephrotoxicity (creatinine > 2 mg/dl) occurred on only 2 cycles (3%). Three patients (11%) developed symptoms of peripheral neuropathy. In 23 evaluable patients, the overall response rate was 39%. CONCLUSION: It is feasible to give 180 mg/m2 of DDP and STS every two weeks with tolerable nephrotoxicity but without blocking other types of toxicity, such as myelosuppression and ototoxicity. The shortening of cycle intervals resulted in a markedly increased DI.

Adult↗

PubMed1993

In vitro modulation of cisplatin accumulation in human ovarian carcinoma cells by pharmacologic alteration of microtubules.

We have previously shown that forskolin and 3-isobutyl-1-methylxanthine (IBMX) increased accumulation of cisplatin (DDP) in DDP-sensitive 2008 human ovarian carcinoma cells in proportion to their ability to increase cAMP. Since the major function of cAMP is to activate protein kinase A, it was conjectured that the stimulation of DDP accumulation was mediated by a protein kinase A substrate. We now show that exposure of 2008 cells to forskolin resulted in phosphorylation of a prominent 52-kD membrane protein. Microsequencing of the band demonstrated it to be human beta-tubulin. Similarly, pretreatment of 2008 cells with the microtubule stabilizing drug taxol increased platinum accumulation in a dose-dependent manner. In 11-fold DDP-resistant 2008/C13*5.25 cells, decreased DDP accumulation was associated with enhanced spontaneous formation of microtubule bundles and decreased expression of beta-tubulin and the tubulin-associated p53 antioncogene relative to 2008 cells. 2008/C13*5.25 cells had altered sensitivity to tubulin-binding drugs, being hypersensitive to taxol and cross-resistant to colchicine. We conclude that pharmacologic alterations of tubulin enhance accumulation of DDP, and that the DDP-resistant phenotype in 2008/C13*5.25 cells is associated with tubulin abnormalities.

Amino Acid Sequence↗

PubMed1993

Extended CSF cytarabine exposure following intrathecal administration of DTC 101.

PURPOSE: The aims of the present study were to determine ventricular and lumbar CSF pharmacokinetics and the maximum-tolerated dose (MTD) of DTC 101 (DepoFoam; DepoTech Corp, La Jolla, CA) following intraventricular administration. PATIENTS AND METHODS: Twelve patients with neoplastic meningitis were treated with escalating doses of DTC 101. CSF samples were obtained from the right lateral ventricle or from the lumbar subarachnoid space and cytarabine concentrations were determined by high-performance liquid chromatography. RESULTS: Therapeutic ventricular CSF concentrations were maintained for 9 +/- 2 days following administration of a single dose of DTC 101 into the lateral ventricle. Lumbar cytarabine concentrations became equal to those in the ventricle within the first 6 hours after intraventricular injection, and the subsequent decay in concentrations of free and total cytarabine were the same at both sites. Following intralumbar administration, the peak ventricular concentration of free cytarabine was reached within 1 day, and therapeutic ventricular CSF levels were maintained for several days. Therapeutic intralumbar concentration of free cytarabine was maintained for up to 14 days. The MTD was 75 mg of DTC 101, and seven of nine patients manifested cytologic responses. CONCLUSION: Extended CSF exposure to therapeutic cytarabine concentrations was achieved after a single intraventricular or intralumbar dose of DTC 101, permitting drug administration once every 2 weeks.

Adolescent↗

PubMed1993

Correlation of the survival of ovarian cancer patients with mRNA expression of the 60-kD heat-shock protein HSP-60.

PURPOSE: Ovarian carcinomas express the 60-kD heat-shock protein HSP-60 at widely varying levels in different tumors. The aim of this study was to determine whether there was a relationship between expression of HSP-60 and survival in patients with ovarian carcinoma. MATERIALS AND METHODS: Total RNA and DNA were prepared from 51 epithelial ovarian cancer tissue samples. The expression and structure of the HSP-60 gene were examined by Northern and Southern blot analyses using the carboxyl-terminal portion of this gene as a probe (0.89 kilobases [kb]). HSP-60 expression was correlated with overall survival by the Kaplan-Meier method. RESULTS: The 2.3-kb HSP-60 message was detected in all samples, but there was marked variation from tumor to tumor. Patients were classified into two groups on the basis of HSP-60 expression: group 1 (n = 25) included patients with low expression, and group 2 (n = 26) consisted of patients with high expression. There were no significant differences between the groups with respect to age, cell type, pathologic grade, clinical stage, and previous treatment. After a median follow-up period of 17 months, Kaplan-Meier plots demonstrated a much better survival for group 1 (median, 46.8 months; 41% at 4 years) than group 2 (median, 22.1 months; 16% at 3.9 years), a difference that was highly significant by the Mantel-Haenszel test (P = .00183). Southern blot analysis of these samples showed no amplification or rearrangement of the gene. CONCLUSION: The level of HSP-60 mRNA expression is a valuable prognostic factor in epithelial ovarian cancer. Variation in the level of expression is not due to amplification of this gene.

Adult↗

PubMed1993

Profiles of women age 30-39 and age less than 30 with epithelial ovarian cancer.

OBJECTIVE: To study a group of women diagnosed with epithelial ovarian cancer at a young age (less than 40). METHODS: Tumor registry data were analyzed with respect to age at diagnosis, stage, grade, frequency of nulligravidity, and family history of breast or ovarian cancer. Frequencies were analyzed using contingency tables, and survival distributions were analyzed according to the method of Kaplan and Meier. Multivariate survival analysis was performed with the Cox method. RESULTS: We found significantly higher frequencies of low-grade tumors (90 versus 37%; P = .0003, chi 2 test) and early-stage tumors (45 versus 17%; P = .03, Fisher exact test) in women less than 30 at the time of diagnosis (very young patients) than in those between 30-39. We also found a significant (P = .017, Breslow statistic) survival advantage for the very young women. Multivariate analysis demonstrated tumor grade as the independent variable for survival. CONCLUSION: These findings support the concept of a preclinical phase of epithelial carcinoma and show that young women may derive substantial benefit from ovarian cancer screening programs.

Adult↗

PubMed1992

Modulation of cisplatin resistance in human malignant melanoma cells.

Previous studies have shown that the combination of dacarbazine, carmustine, cisplatin (DDP), and tamoxifen (TAM) produced a 53% overall response rate in patients with disseminated melanoma. Deletion of TAM from the regimen resulted in a fall in the response rate to 10%, and reincorporation of TAM returned the response rate to 52%, suggesting an important role for TAM. Using the human melanoma cell line T-289, we examined the nature of the interaction between TAM and each member of this combination in clonogenic assays in soft agar. The combination of TAM with DDP was highly synergistic as demonstrated by median effect analysis, whereas TAM was antagonistic with carmustine and an activated form of dacarbazine. The mean combination index at 50% kill was 0.26 +/- 0.02 (mean +/- SD) for TAM and DDP. This marked synergism was observed at concentrations of TAM that are clinically achievable. TAM had no effect on the uptake of the DDP analogue [3H]dichloro(ethylenediamine)platinum(II). There was no effect on the formation or repair of DDP intrastrand DNA adducts. Similarly, there was no effect demonstrated on the intracellular concentrations of glutathione or metallothioneins. We conclude that the interaction between TAM and DDP is truly synergistic in this cell line and is accomplished through none of the four mechanisms commonly associated with DDP resistance.

Carboplatin↗